In the case of currently used networks, as generally known in the art, networks for broadcasting services are separate from those for data services. Broadcasting services are provided via coaxial cables, satellites, or terrestrial waves. Data services are available via xDSL or cable modems, which are based on copper wires, so that Internet services can be provided at a rate of tens of kbps or a number of Mbps. Recently, extensive use of Internet and rapid replacement of conventional services, which have mainly been developed for voice and text communication, by video-oriented services have resulted in ever-increasing request for faster networks. This means that conventional networks need upgrade or replacement. However, it costs a large amount of money to install networks for broadcasting services separate from networks for data services, and furthermore, separate maintenance and management of networks are unfavorable from an economic point of view. In an attempt to solve these problems, much effort is being made to provide a combined system capable of providing both broadcasting and communication data services. Meanwhile, networks based on copper wires can hardly satisfy the demand for higher rate, due to their physical limitations.
Lately passive optical networks (hereinafter, referred to as PONs) based on optical fiber are drawing much attention. This is because PONs can provide both data and broadcasting services while satisfying the requests for faster networks and combination of broadcasting and data services. According to the implementation method, PONs are classified into TDM (Time Division Multiplexing)-PONs and WDM (Wavelength Division Multiplexing)-PONs. In terms of provision of broadcasting services, TDM-PONs have limitations regarding the number of subscribers and the transmission distance, because their optical splitters have serious losses. Amplifiers may be used to compensate for such limitations. However, when optical signals having a predetermined level or more of output power are inputted to optical fiber, the nonlinearity of the optical fiber increases noise components and results in a penalty. When it comes to data transmission, TDM-PONs have a fixed downstream transmission rate, which is shared by a number of subscribers. This means that, as the number of subscribers (i.e. branches) increases, the transmission rate for each subscriber is decreased. In addition, a complicated MAC (Media Access Control) protocol is necessary to solve problems of ranging related to the distance between subscribers. And other problems including implementation of burst-mode optical receivers, security based on the broadcasting mode must be solved. This increases the system complexity. Particularly, the splitting loss occurring in the optical splitters worsens the optical loss from the telephone office to the subscribers. This is a major obstacle to increase the transmission rate.
In contrast, WDM-PONs can guarantee transparency regarding the protocol and transmission rate, flexibility for accommodating various services, and excellent network expandability. In addition, the fact that each subscriber has point-to-point connection with the central office secures the quality of service and ensures a high level of security and privacy. As such, WDM-PONs are regarded as the ultimate optical networks due to the merit of accommodating services with various rates and modulation formats for respective subscribers. Consequently, it is increasingly requested to develop a WDM-PON for providing a combined service of broadcasting and communication.
FIG. 1 shows a first example of a WDM-PON adapted to simultaneously transmit broadcasting and communication services according to the prior art. Referring to FIG. 1, a central office (hereinafter, referred to as CO) 100 consists of a plurality of first optical transmitters 110a for providing a plurality of optical network terminals (hereinafter, referred to as ONTs) 500 with a downstream broadcasting service and a plurality of second optical transmitters 110b for providing a data service. The first and second optical transmitters 110a and 110b use different wavelengths. A WDM filter 130a multiplexes optical signals for broadcasting via the same channel and optical signals for data. An arrayed waveguide grating (hereinafter, referred to as AWG) 140 multiplexes optical signals of a plurality of channels. The optical signals are demultiplexed for respective channels as they pass through an optical fiber 200 and an AWG 310. Then, the demultiplexed optical signals are separated into optical signals for the broadcasting service and optical signals for the data service by an optical fiber 400 and a WDM filter 530. Finally, the optical signals are transmitted to first and second optical receivers 520a and 520b, respectively, and are converted into electrical signals.
In order to provide both broadcasting and data communication services, two downstream optical transmitters and two downstream optical receivers are necessary for each subscriber. This increases the price of the WDM-PON system and, as a result, the burden on subscribers.
FIG. 2 shows a second example of a WDM-PON adapted to simultaneously transmit broadcasting and communication services according to the prior art. Referring to FIG. 2, the optical transmitters (FIG. 1) for providing a broadcasting service are replaced with a BLS. The CO 100 consists of a plurality of optical transmitters 110c for providing a plurality of ONTs 500 with a downstream data service. Optical signals of a plurality of channels for a data service pass through a WDM filter 130b and an AWG 140 and are multiplexed. Then, the multiplexed optical signals are added to a BLS 160 for providing a broadcasting service via a WDM filter 180. In order to apply a broadcasting signal, the BLS is modulated through a direct or external modulation process.
FIG. 3 shows the spectra of a BLS before and after it passes through an AWG according to the prior art. Referring to FIG. 3 together with FIG. 2, a spectrum of the BLS 160 before it passes through the AWG 310 is labeled 600. After passing through the AWG 310, the spectrum 600 is divided into spectrums 610, 620, and 630. Particularly, the broad spectrum 600 of the BLS 160 is spectrally sliced according to the wavelength of an output port, which the AWG 310 passes. After the spectrum division, the spectrally sliced light is transmitted from each output port of the AWG 310 so that the broadcasting service is available to each subscriber. The procedure after the AWG 310 is the same as in the case of FIG. 1.
The BLS 160 shown in FIG. 2 may be a light source using amplified spontaneous emission light, such as LED (Light Emitting Source) or EDFA (Erbium Doped Fiber Amplifier). The LED does not have sufficient power, and the EDFA increases the budget. These light sources output incoherent light, and the resulting beating noise degrades the signal quality. This limits the number of broadcasting channels available for transmission. The noise depends on the filtered bandwidth. In the case of a Gaussian filter having a bandwidth of 50 GHz, its RIN (Relative Intensity Noise) is −108.8 dB/Hz. Furthermore, use of an external modulator allows polarization to pass in only one direction. As a result, the RIN is increased by 3 dB and becomes −105.8 dB/Hz.
The second example shown in FIG. 2 employs a single BLS so as to providing a broadcasting service. This is advantageous from an economic point of view. However, the light source has a high RIN value due to the beating noise. This reduces the number of channels available for transmission and makes it impossible to transmit signals in a format requiring a high SNR.
FIG. 4 shows a BLS depending on polarization using mutually injected F-P LDs according to the prior art. FIG. 5 shows a BLS not depending on polarization using mutually injected F-P LDs according to the prior art. FIG. 6 shows an output spectrum of a BLS using mutually injected F-P LDs according to the prior art. FIG. 7 shows noise characteristics of a BLS using mutually injected F-P LDs according to the prior art.
FIG. 7 shows the RIN of a mode of a BLS, which has been obtained by mutually injecting F-P LDs having anti-reflective coating. It is clear from the drawing that, except for the low-frequency domain, the RIN has a very low value of about −135 dB/Hz. Even if this is applied to a wavelength-locked F-P LD proposed by Registered Korean Patent No. 325, 687 (Feb. 8, 2002), entitled “WAVELENGTH DIVISION MULTIPLEXING LIGHT SOURCE FOR OPTICAL COMMUNICATION USING FABRY-FEROT LASER DIODE WAVELENGTH-LOCKED BY INJECTED INCOHERENT LIGHT,” the noise characteristics are similar to those of the BLS using mutually injected F-P LDs.